Power conversion apparatus and power conversion method

ABSTRACT

A power conversion apparatus and a power conversion method are provided. The power conversion apparatus includes a rectifier configured to convert AC power to DC power, a smoothing filter configured to control the DC power received from the rectifier to be constant, an inverter configured to convert the DC power received from the smoothing filter into high-frequency power by turning the DC power on and off using a switching device, and a control unit configured to control the rectifier and the inverter. A rating of output power from the inverter is determined in accordance with a frequency of the high-frequency power output from the inverter, a current-applying time, and an operation rate obtained by dividing the current-applying time by a sum of the current-applying time and a non-current-applying time.

TECHNICAL FIELD

The present invention relates to a power conversion apparatus and apower conversion method.

BACKGROUND ART

Electric heating methods for heat treatment include induction heatingand direct electric heating. In hardening treatment in particular whichis one kind of heat treatment using induction heating, a properfrequency is selected according to a heat treatment depth in aworkpiece.

According to a related art power conversion apparatus for heattreatment, DC power is converted into high-frequency power by performingswitching using a power semiconductor device (see, e.g., pamphlet ofMK16A Transistor Inverter, Neturen Co., Ltd.,www.k-neturen.co.jp/eng/business/induction/pdf/MK16Aenglish.pdf).Examples of the power semiconductor switching device includes athyristor for lower than 10 kHz, an IGBT for 10 kHz to 100 kHz, and aMOSFET for higher than 100 kHz.

Comparing the oscillation frequency of 10 kHz and the oscillationfrequency of 100 kHz, the power semiconductor device undergoes muchdifferent temperature increase because of the 10 times difference in theswitching frequency. That is, if a capacity (maximum rated value) of aninverter of the power conversion apparatus is set based on the maximumfrequency of an operable range of the apparatus, the temperatureincrease is small when the output frequency is low and hence theoperation is not economical.

In a power conversion apparatus having a power semiconductor device asdescribed above, the temperature is controlled such that the junctiontemperature of the power semiconductor device does not exceed a giventemperature. More specifically, a thermostat is attached to theperiphery of the power semiconductor device or a thermistor isincorporated in the power semiconductor device. Output of power iscontrolled or suspended only after the actual temperature has reachedthe given temperature.

Another related art power conversion apparatus includes a semiconductordevice for performing a power conversion, heat radiation fins forradiating heat generated by the semiconductor device, a cooling fan forcooling the radiation fins, a detection unit for detecting a parameterrelating to the cooling performance of the cooling fan, and a controlunit (see, e.g., JP2012-39745A). In this power conversion apparatus, ajunction temperature of the semiconductor device is estimated based on adetection result of the detection unit, a loss of the semiconductordevice, and an ambient temperature of the semiconductor device, and thesemiconductor device is controlled such that the estimated junctiontemperature does not exceed a given temperature.

However, according to the temperature controlling method describedabove, because the output of power is suspended only after the actualtemperature has reached the given temperature, a rapid temperatureincrease cannot be addressed due to a response delay of a temperaturesensor, and may lead to a damage of the semiconductor device. Accordingto the related art power conversion apparatus described above, thesemiconductor device is air-cooled, and the junction temperature of thesemiconductor device is estimated taking into account the parameterrelating to the air cooling performance. Further, the temperature sensoris attached to the periphery of the semiconductor device, and thejunction temperature of the semiconductor device is estimated based onthe temperature detected by the temperature sensor. However, themeasured temperature value largely varies depending on the position atwhich the temperature sensor is attached. Accordingly, this related artcannot control the semiconductor device with sufficient accuracy.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an economical powerconversion apparatus and power conversion method for heat treatmentwhich can vary the output within a rated value in accordance with a usecondition.

According to an aspect of the present invention, a power conversionapparatus for heat treatment is provided. The power conversion apparatusincludes a rectifier configured to convert AC power to DC power, asmoothing filter configured to control the DC power received from therectifier to be constant, an inverter configured to convert the DC powerreceived from the smoothing filter into high-frequency power by turningthe DC power on and off using a switching device, and a control unitconfigured to control the rectifier and the inverter. A rating of outputpower from the inverter is determined in accordance with a frequency ofthe high-frequency power output from the inverter, a current-applyingtime, and an operation rate obtained by dividing the current-applyingtime by a sum of the current-applying time and a non-current-applyingtime.

The control unit may have data defining a relationship among a frequencyof the switching device, the current-applying time, the operation rate,and the output power at a temperature at which the switching device isoperable, and when the current-applying time and the operation rate aregiven, the control unit may calculate a maximum allowable current basedon the data and may suspend or control the output power.

According to another aspect of the present invention, a power conversionmethod for heat treatment is provided. The power conversion methodincludes converting AC power into DC power and converting the DC powerinto high-frequency power by turning the DC power on and off using aswitching device. The power conversion method further includesincreasing maximum output power in accordance with a frequency of thehigh-frequency power output from the inverter, a current-applying time,and an operation rate obtained by dividing the current-applying time bya sum of the current-applying time and a non-current-applying time, suchthat a junction temperature of the switching device does not exceed agiven temperature.

The maximum output power may be increased until an upper limit of thejunction temperature of the switching device reaches a rated temperatureof the switching device, the junction temperature being obtained fromthe difference between a temperature increase due to a loss of theswitching device and a temperature decrease due to cooling of theswitching device.

The loss of the switching device may be determined based on the sum of aconduction loss of the switching device and a switching loss of theswitching device.

The time during which the electric current is applied to the switchingdevice is is shorter than the time for changing and setting a workpieceto be heat-treated.

According to the apparatus and the method described above, since therating is determined in accordance with the frequency, thecurrent-applying time, and the operation rate, the output can beincreased within the rating of the switching device when converting intoa low frequency power. Accordingly, economical apparatus and method canbe provided.

It is another object of the present invention to provide a powerconversion apparatus and a power conversion method which can preventdamaging of a power semiconductor device even when there is a change inan output condition.

According to another aspect of the present invention, a power conversionapparatus includes a power conversion unit, a plurality of sensorsattached to the power conversion unit, and a control unit configured tocontrol the power conversion unit. The power conversion unit includes amodule having a power semiconductor device to perform a power conversionand a metal base on which the power semiconductor device is mounted, anda heat sink arranged to contact the metal base to cool the powersemiconductor device. The sensors are arranged to measure a temperatureof the metal base and a temperature and a flow rate of cooling waterflowing into and out of the heat sink. The control unit is configured toestimate an initial junction temperature of the power semiconductordevice by obtaining, based on values measured by the sensors, a quantityof heat that flows from the power semiconductor device to the heat sink.The control unit is configured to further obtain, upon receipt of anoutput change instruction to increase power to be output from the powerconversion unit, an updated junction temperature of the powersemiconductor device corresponding to increased power in accordance withthe output change instruction. The control unit is configured to becomenon-responsive to the output change instruction when the control unitdetermines that the updated junction temperature reaches a giventemperature.

The control unit may have device data indicating a relationship betweenapplied electric current and voltage of the power semiconductor devicefor each junction temperature. When obtaining the updated junctiontemperature of the power semiconductor device, the control unit mayobtain voltage corresponding to an increased applied electric currentfrom the device data for the initial junction temperature to obtain,based on the increased applied electric current and the correspondingvoltage, a power loss and the updated junction temperature correspondingto the power loss. Then, the control unit may repeat the steps ofobtaining updated voltage corresponding to the increased appliedelectric current from the device data for the latest updated junctiontemperature and obtaining, based on the increased applied electriccurrent and the updated voltage, updated power loss and another updatedjunction temperature corresponding to the updated power loss.

The plurality of sensors may include a temperature sensor configured tomeasure the temperature of the metal base. The temperature sensor may beprovided to contact the metal base or inserted in the metal base. Thecontrol unit may estimate the initial junction temperature of the powersemiconductor device based on a thermal resistance circuit between ajunction of the power semiconductor device and the metal base.

According to yet another aspect of the present invention, a powerconversion method is provided. The power conversion method uses a modulehaving a power semiconductor device and a metal base on which the powersemiconductor device is mounted, and a heat sink arranged to contact themetal base to cool the power semiconductor device. Power conversion isperformed by an operation of the power semiconductor device. The powerconversion method includes estimating an initial junction temperature ofthe power semiconductor device by measuring a quantity of heat thatflows from the power semiconductor device to the heat sink when electriccurrent is applied to the power semiconductor device. The powerconversion method further includes obtaining, when the electric currentapplied to the power semiconductor device is to be increased, an updatedjunction temperature of the power semiconductor device based on anincreased applied electric current. The power conversion method furtherincludes withholding an increase of the electric current applied to thepower semiconductor device when it is determined that the updatedjunction temperature reaches a given temperature.

The obtaining the updated junction temperature of the powersemiconductor device may include obtaining voltage corresponding to theincreased applied electric current from device characteristics of thepower semiconductor device based on the initial junction temperature toobtain, based on the increased applied electric current and thecorresponding voltage, a power loss and the updated junction temperaturecorresponding to the power loss, and then repeating a series of steps ofobtaining an updated voltage corresponding to the increased appliedelectric current from the device characteristics of the powersemiconductor device based on the latest updated junction temperature,and obtaining, based on the increased applied electric current and theupdated voltage, an updated power loss and another updated junctiontemperature corresponding to the updated power loss. The series of stepsmay be repeated until the updated junction temperature converges.

According to the power conversion apparatus described above, the controlunit estimates the initial junction temperature of the powersemiconductor device by obtaining the quantity of heat that flows fromthe power semiconductor device to the heat sink based on the temperaturedifference and the flow rate of the cooling water. Upon receipt of theoutput change instruction to increase the output, the control unitobtains a temperature increase of the power semiconductor deviceassuming that the power conversion unit is controlled in response to theoutput change instruction, and does not allow an output that is inaccordance with the output change instruction if the updated junctiontemperature exceeds a given temperature. That is, the control unitcontrols or suspends the output so that the temperature of the junctionof the power semiconductor device does not exceed the given temperature.Thus, the power semiconductor device of the power conversion unit can beprevented from being damaged by heat generated itself.

According to the power conversion method described above, the initialjunction temperature of the power semiconductor device is estimated bymeasuring the quantity of heat that flows from the power semiconductordevice to the heat sink when the electric current is applied to thepower semiconductor device. When the electric current applied to thepower semiconductor device is to be increased, the updated junctiontemperature of the power semiconductor device corresponding to theincreased applied electric current is obtained. The electric currentapplied to the power semiconductor device is not increased if it isdetermined that the calculated junction temperature exceeds a giventemperature. Thus, the power semiconductor device can be prevented frombeing damaged by heat generated by itself.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a power conversionapparatus according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating operation rate.

FIG. 3 is diagram illustrating a portion of data stored in a controlunit.

FIGS. 4A and 4B are diagrams illustrating a design concept of the powerconversion apparatus according to an embodiment of the presentinvention, FIG. 4A illustrating a collector current waveform and acollector-emitter voltage waveform of a switching device, and FIG. 4Billustrating a loss waveform.

FIGS. 5A to 5C are diagrams illustrating a method for calculating ajunction temperature of the switching device based on a regularlyrepeating rectangular pulse current.

FIGS. 6A and 6B are diagrams illustrating data stored in the controlunit, showing relationships between the operation rate and the power at3 kHz and 50 kHz, respectively, with the current-applying time being aparameter.

FIG. 7 is a diagram illustrating a configuration of a power conversionapparatus according to another embodiment of the present invention.

FIG. 8 is a diagram illustrating a thermal resistance circuit from apower semiconductor device to cooling water via a metal base and a heatsink in the power conversion apparatus shown in FIG. 7.

FIG. 9 is a diagram showing data stored in a control unit shown in FIG.7, indicating characteristics of the power semiconductor device.

FIG. 10 is a diagram illustrating a method by which the control unitestimates a junction temperature of the power semiconductor device inchanging the output in response to an output change instruction.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described indetail with reference to FIGS. 1 to 6B.

FIG. 1 is a diagram illustrating a configuration of a power conversionapparatus according to an embodiment of the present invention. As shownin FIG. 1, a power conversion apparatus 10 for heat treatment(hereinafter, the power conversion apparatus 10) includes a rectifier 11configured to convert AC power to DC power, a smoothing filter 12configured to control the DC power received from the rectifier 11 to beconstant, an inverter 13 configured to convert the DC power receivedfrom the smoothing filter 12 into high-frequency power by turning aswitching device on and off at a given frequency, and a control unit 14configured to control the rectifier 11 and the inverter 13.

The rectifier 11, also called a converter, converts the commercial ACpower into the DC power by rectifying the commercial AC power. Therectifier 11 adjusts the magnitude of the output power of the powerconversion apparatus 10 under output control by the control unit 14.

Where the power conversion apparatus 10 is of a current type, thesmoothing filter 12 smoothes out ripples in an electric current outputfrom the rectifier 11 by means of a reactor, and outputs the resultingcurrent to the inverter 13. Where the power conversion apparatus 10 isof a voltage type, the smoothing filter 12 smoothes out ripples in avoltage output from the rectifier 11 by means of a capacitor, andoutputs the resulting voltage to the inverter 13.

The power semiconductor device as the switching device is configured toform a bridge circuit, and by the switching of the power semiconductordevice, the inverter 13 converts the DC power into high-frequency powerand outputs the high-frequency power.

The control unit 14 controls the rectifier 11 by sending an outputcontrol signal and an abnormal stop instruction signal to rectifier 11,and controls the inverter 13 by sending a frequency control signal andan abnormal stop instruction signal to the inverter 13. The control unit14 receives feedback signals from the rectifier 11 and the inverter 13respectively, and detects the conditions of the rectifier 11 and theinverter 13.

According to an embodiment of the invention, the control unit 14suspends or controls the output from the inverter 13 in accordance withthe frequency of the high-frequency power output from the inverter 13,the current-applying time, and the operation rate. To this end, thecontrol unit 14 has data defining a relationship among the frequency ofthe switching device, the current-applying time, the operation rate, andthe output power at a temperature at which the switching device isoperable. When the current-applying time and the operation rate aregiven, the control unit 14 calculates a maximum allowable current basedon the above data. When an output current of the rectifier 11 detectedby a current feedback signal received from the rectifier 11 exceeds themaximum allowable current that is calculated based on thecurrent-applying time, the operation rate, and the frequency, thecontrol unit 14 suspends or lowers the output from the inverter 13 Themaximum allowable current is calculated based on the data that definesthe relationship among the frequency of the switching device, thecurrent-applying time, the operation rate, and the power at atemperature at which the switching device is operable, and is themaximum current that is allowed to flow under the given conditions. Inthis manner, even during operation of the power conversion apparatus 10,the control unit 14 suspends or controls the output of the powerconversion apparatus 10 based on a temperature increase caused by theoperation of the switching device, with the maximum output power, i.e.,the capacity of the inverter 13 being rated.

The operation rate α will be described below with reference to FIG. 2.In FIG. 2 the horizontal axis represents time and the vertical axisrepresents output. The operation rate α is given by the followingequation.

Operation  Rate  α = Current-Applying  Time  tp/Cycle  τ = Current-Applying  Time  tp/(Current-Applying  Time + Non-Current-Applying  Time)

The current-applying time tp is a time during which high-frequency poweris output from the inverter 13. The cycle τ is the sum of thecurrent-applying time and the non-current-applying time, and is a timefrom an output of one pulse to an output of the next pulse.

Data stored in the control unit 14 indicates a relationship of the powerthat causes a junction temperature increase ΔTj of the switching devicewith respect to the operation rate α, the current-applying time tp, andthe frequency. FIG. 3 is a diagram illustrating a portion of the datastored in the control unit 14. In FIG. 3, the horizontal axis representsthe operation rate α (%) and the vertical axis represents the power(kW). When the operation rate is 100%, the power is fixed to P₁, therebyproviding a continuous rating. When the operation rate decreases, thepower increases. When the current-applying time is shortened, theincrease amount of the power becomes larger.

The design concept of the power conversion apparatus 10 according to anembodiment of the invention will be described with reference to FIGS. 4Aand 4B. FIG. 4A shows a collector current I_(C) waveform and acollector-emitter voltage V_(CE) waveform of the switching device, andFIG. 4B shows a loss waveform. In FIGS. 4A and 4B, both horizontal axesrepresent time t. The rating of the output of the power conversionapparatus 10 is determined by temperature characteristics of theswitching device and other characteristics such as a rated voltage andtemperature balance. The temperature of the switching device isdetermined by the loss and the cooling of the switching device. The lossof the switching device is given by the following.

Loss of Device=Steady Loss +Switching Loss

As shown in FIGS. 4A and 4B, when the collector current I_(C) and thecollector-emitter voltage V_(CE) are shown with the horizontal axisbeing the time, the collector current I_(C) leads the collector-emittervoltage V_(CE) in phase. This phase shift between these waveforms causesa switching loss by an amount corresponding to the product of thecollector current I_(C) and the collector-emitter voltage V_(CE).

The steady loss is a loss that is caused by an application of electriccurrent to the switching device, that is, a conduction loss of theswitching device, and depends on a value of the electric current to beapplied. On the other hand, the switching loss is proportional to thenumber of switching (i.e., frequency). Therefore, even with the samecurrent, the switching loss increases and hence the device lossincreases as the frequency becomes higher.

In spite of this fact, conventional power conversion apparatuses, bothpower conversion apparatuses for high oscillation frequencies and powerconversion apparatuses for low oscillation frequencies are rated with amaximum frequency at which the loss is the highest and with anassumption of a continuous operation. This means that, when a frequencyis low in a power conversion apparatus for high frequencies, low currentis caused to flow although higher current can be set. Furthermore, thefact is not taken into consideration that there exist sufficient timesfor cooling of the switching device in the case where very shortenergization (e.g., several seconds to a little more than 10 seconds),instead of continuous energization, is done as in the case ofhigh-frequency hardening, for example.

In view of the above, according to an embodiment of the presentinvention, for each frequency, the output power as the rating isdetermined taking the cooling time of the switching device intoconsideration based on the current-applying time and the operation rate.That is, the control unit 14 determines, for each oscillation frequencyof the power conversion apparatus, output power by calculating a currentat which the junction temperature does not exceed a given temperatureaccording to a current-applying time tp and an operation rate α on thebasis of the device characteristics of the switching device used in theinverter 13. If the output current of the inverter 13 becomes largerthan a reference current, the control unit 14 suspends operation of therectifier 11 and the inverter 13 and suspends output from the inverter13. in this manner, the output of the power conversion apparatus israted for each of frequencies that are set finely, taking an operationrate and a current-applying time into consideration. This makes itpossible to make good use of margins that are available in a lowfrequency range.

According to the power conversion method of the embodiment describedabove, when converting AC power into DC power and then converting the DCpower into high-frequency power by turning the DC power on and off usingthe switching device, the maximum output power is increased inaccordance with the frequency after the conversion, the current-applyingtime, and the operation rate obtained by dividing the current-applyingtime by the sum of the current-applying time and thenon-current-applying time, within a range in which the junctiontemperature of the switching device does not exceed the given value.

A junction temperature of the switching device is obtained from thedifference between a temperature increase due to the loss of theswitching device and a temperature decrease due to the cooling of theswitching device, and the maximum output power is increased until theupper limit of the junction temperature of the switching device reachesthe rated temperature of the switching device. This makes it possible torealize economical heat treatment. In particular, economic efficiency isremarkably improved because the time during which the electric currentis applied to the switching device is much shorter than the time forchanging and/or setting a workpiece to be heat-treated.

Next, an example calculation method of a reference current value basedon which the control unit 14 suspends the output. FIGS. 5A to 5Cillustrate a method for calculating a junction temperature Tj of theswitching device based on a current in which rectangular pulses occurrepeatedly and regularly. As shown in FIG. 5A, the current-applying timeof the power loss Ptm is represented by tp, and the cycle thereof isrepresented by τ, respectively. A temperature increase is calculated bymaking power loss approximation by averaging power losses of pulsesother than the two latest ones (see FIG. 5B) and applying the principleof superposition to the power loss calculation (see FIG. 5C).

The junction temperature Tj of the switching device is calculated on thebasis of current in which rectangular pulses occur repeatedly andregularly according to the following equation

Tj=Tw+Ptm{(tp/τ)·R(j−w)+(1−tp/τ)·R(j−w)(τ+tp)−R(j−w)(τ)+R(j−w)(tp)}.

This equation is modified as follows.

Tj−Tw=(T∞+T3−T2+T1)·Ptm

T∞=(tp/τ)·R(j−w)

T3=(1−tp/τ)·R(j−w)(τ+tp)

T2=R(j−w)(τ)

T1=R(j−w)(tp).

T∞ means that the loss Ptm occurs with the current-applying rate tp/τfor infinite time, and is given by a thermal resistance at the time of acontinuous rating multiplied by the current-applying rate tp/τ.

T3 means that a part corresponding to the current-applying rate tp/τ issubtracted from the loss of the time (τ+tp).

−T2 means that a loss of a time τ is subtracted.

T1 means that a loss of a time tp is added.

τ denotes the cycle time, and R(j−w)(t) denotes a transient thermalresistance (° C./W) of a time t. Tw denotes the temperature (° C.) ofcooling water.

The junction temperature Tj is calculated in the manner described above.When the junction temperature of the switching device of the inverter 13reaches the reference value, the control unit 14 suspends the operationof the rectifier 11 and the inverter 13 and thus controls the output.This is because when the switching device is operated, loss is producedby applying the electric current, and when the junction temperaturebecomes higher than the reference temperature, the switching devicemight be broken. The loss is, for example, obtained from the sum of asteady loss and a switching loss which are calculated in the followingmanners.

The steady loss is calculated by determining a loss value at a certaincurrent in advance and multiplying the loss value by a loss increasecoefficient due to current increase and a loss increase coefficient ofthe device due to the current increase. On the other hand, the switchingloss is calculated by determining a loss value per kilohertz in advanceand multiplying it by a frequency with additional consideration of afactor relating to current increase.

A relationship is maintained that a temperature obtained by multiplyingthe sum of a thus-calculated steady loss and switching loss by(T∞+T3−T2+T1) is lower than or equal to a given temperature.

Since the given temperature is determined for the switching device,determining a current (called a reference current) that satisfies theabove relationship makes it possible to increase the output within sucha range that the current flowing through the switching device does notexceed the reference current.

Results that were obtained by the above method will be described below.FIGS. 6A and 6B are diagrams illustrating data that are stored in thecontrol unit 14 and indicate a relationship between the operation rate αand the power at 3 kHz and 50 kHz, respectively, with thecurrent-applying time tp being a parameter. In FIGS. 6A and 6B, t1 tot4, which are values of tp, satisfy a relationship of t1<t2<t3<t4.

In power conversion apparatus for heat treatment, in the case ofoutputting high-frequency power at the frequency of 3 kHz, the rating isdetermined as shown in FIG. 6A. The power can be increased by decreasingthe operation rate α. For each of the current-applying times t2 to t4,the output power can be varied according to the operation rate. However,in the case of the current-applying time t1, the output power cannot beincreased even if the operation rate is decreased below a certain value.

In the case of outputting high-frequency power at the frequency of 50kHz, the rating is determined as shown in FIG. 6B. The power can beincreased by decreasing the operation rate α. For each of thecurrent-applying times t1 to t4, the output power can be variedaccording to the operation rate.

In power conversion apparatus produced according to the same designconcept as described above, the output power can be varied according tothe frequency and more power can be output as the frequency decreases.

Therefore, in the power conversion apparatus according to the embodimentof the invention, the individual rating is set for each outputfrequency. Conventionally, the rating of a high frequency powerconversion apparatus has been the same as the rating of a low frequencypower conversion apparatus. In contrast, according to the embodiment ofthe invention, its economic efficiency can be improved by setting alarge rating for a low frequency in accordance with the rating of thepower conversion apparatus. Depending on the output frequency, it may benecessary to replace the rectifier 11, the inverter 13 or the controlunit 14 or to change the constants of these components. However, theoscillation frequency can be changed by making such fine adjustments byperforming a switching using a switch.

Next, another embodiment of the present invention will be described withreference to FIGS. 7 to 10.

FIG. 7 is a diagram illustrating a configuration of a power conversionapparatus 10A according to another embodiment of the invention. As shownin FIG. 7, a power source 31 is coupled to an input side of the powerconversion apparatus 10A, and a load 32 is coupled to an output side ofthe power conversion apparatus 10A. The power conversion apparatus 10Aincludes a power conversion unit 11 and a control unit 14A configured tocontrol the power conversion unit 20.

The power conversion unit 20 includes a module 23 having a powersemiconductor device 21 for power conversion and a metal base 22 onwhich the power semiconductor device 21 is mounted, and a heat sink 24arranged to contact the metal base 22 of the heat sink 24 to cool thepower semiconductor device 21. In the module 23, an insulting layer 25is sandwiched between the power semiconductor device 21 and the metalbase 22. The power semiconductor device 21 inside one or more modules 23forms a converter or an inverter so that the power conversion unit 20produces power to be output to the load 32 through conversion. The heatsink 24 here is so-called a water-cooling heat sink, and is configuredsuch that a pipe 24 b through which cooling water flows is disposedadjacent to a heat dissipating portion 24 a. Since the heat sink 24 isin contact with the metal base 22, heat generated by the powersemiconductor device 21 can be transferred to the cooling water wefficiently.

A plurality of sensors is attached to the power conversion unit 20. Atemperature sensor 26 a measures a temperature of the metal base 22. Itis preferable that the temperature sensor 26 a be inserted in a recessformed in the metal base 22 or disposed so as to be in contact with themetal base 22. In this manner, the junction temperature can be monitoredirrespective of the attachment position of the temperature sensor 26 a.

Temperature sensors 26 b and 26 c are provided at an inflow-side and anoutflow-side of the pipe 24 b respectively, and measure the temperatureof the cooling water flowing into the heat sink 24 as well as thetemperature of the cooling water flowing out of the heat sink 24. A flowrate sensor 26 d is attached to the pipe 24 d to measure a flow rate ofthe cooling water.

The control unit 14A obtains a quantity of heat that flows from thepower semiconductor device 21 to the heat sink 24 as a temperatureincrease of the cooling water w in accordance with Equation (1) whichwill be described later, and estimates an initial junction temperatureof the power semiconductor device 21 from the actual value measured bythe temperature sensor 26 a. Further, the control unit 14A controls thepower conversion unit 20 upon receipt of an output related instructionfrom an input port (not shown). When the output related instruction isreceived from the input port while the power conversion unit 20 isoutputting power, the control unit 14A determines whether to respond tothe instruction by performing the following processing. Morespecifically, upon receipt of an output change instruction to increasethe power to be output from the power conversion unit 20, the controlunit 14A obtains an updated junction temperature of the powersemiconductor device 21 corresponding to the increased power the outputchange instruction. When the control unit 14A determines that theupdated junction temperature reaches a given temperature, the controlunit 14A becomes non-responsive to the instruction and performs such acontrol as suspending the output. In this manner, thermal destruction ofthe power semiconductor device 21 is prevented.

A description will be made of how the control unit 14A estimates ajunction temperature of the power semiconductor device 21 correspondingto a change of the output in response to the output change instruction.FIG. 8 shows a thermal resistance circuit from the power semiconductordevice to the cooling water via the metal base and the heat sink. Thereare thermal resistance Rth(f−w) between the cooling water (w) and theheat sink (f), thermal resistance Rth(c−f) between the heat sink (f) andthe metal base (c), thermal resistance Rth(j−c) between the metal base(c) and the junctions (j) of the power semiconductor device.

First, at a first step, a current junction temperature (initial junctiontemperature) of the power semiconductor device 21 before changing of theoutput in response to an output change instruction is estimated. Here,it is assumed that the module 23 including the power semiconductordevice 21 is attached to the heat sink 24 and that heat generated by thepower semiconductor device 21 is transmitted to the heat sink 24 anddissipated by the cooling water. By obtaining a temperature increase ofthe cooling water, power from the power semiconductor device 21 due toelectric current being applied, that is, a loss is measured.

That is, a loss P is obtained by multiplying the difference between thetemperature TW(out) of the cooling water w at the outflow-side and thetemperature TW(in) of the cooling water w at the inflow-side by the flowrate according to Equation (1). The coefficient “70” at the end ofEquation (1) reflects characteristics of water of 20° C. as the coolingwater, such as specific heat and density. Symbols enclosed byparentheses [ ] are units. It is assumed that only one module 23 ismounted on the heat sink 24. Where a plurality of modules 23 are mountedon the heat sink 24, an inflow-side temperature and an outflow-sidetemperature of the cooling water may be measured for each module 23 orthe left side of Equation (1) may be modified so as to be the sum oflosses of the respective modules 23.

Loss P [W]={TW(out) [° C.]−TW(in) [° C.]}×flow rate [L/min]×70  (1)

A junction temperature Tj of the power semiconductor device 21 in thethermal resistance circuit shown in FIG. 8 is determined according toEquation (2) on the basis of the power P that has been determinedaccording to Equation (1):

Junction temperature Tj [° C.]=loss P×Rth(j−c)+metal base temperature [°C.]  (2)

In Equation (2), Rth(j−c) is set using a catalog thermal resistancevalue (° C./W) of the power semiconductor device 21.

The current junction temperature Tj of the power semiconductor device 21before changing of the output in response to an output changeinstruction is obtained in accordance with Equation (2).

Subsequently, at a second step, a junction temperature (updated junctiontemperature) of the power semiconductor device 21 at the time ofchanging the output in response to an output change instruction isestimated in accordance with the following. In the following, adescription will be made of an example method for estimating a junctiontemperature of the power semiconductor device 21 at the time ofincreasing the output current from I₁ to I₂ in response to an outputchange instruction.

FIG. 9 is a schematic graph showing data that are stored in the controlunit 14 and indicate a characteristic of the power semiconductor device21. In this example, the power semiconductor device 21 is an IGBT. Thehorizontal axis represents the collector-emitter voltage V_(CE) and thevertical axis represents the collector current I_(C). And the junctiontemperature Tj and the gate voltage V_(G) are parameters. That is,V_(CE)=f(I_(C), Tj, V_(G)). Since the gate voltage V_(G) is constant,the collector-emitter voltage V_(CE) is a function of the collectorcurrent I_(C) and the junction temperature Tj.

When the applied electric current is increased from I₁ to I₂ in thepower semiconductor device 21 having a junction temperature value Tj0,the I_(C)−V_(CE) curve varies depending on the junction temperature Tj.Therefore, first, an I_(C)−V_(CE) curve corresponding to the junctiontemperature value Tj0 before the intended current increase (initialvalue) is used. It is seen from the I_(C)−V_(CE) curve corresponding tothe junction temperature value Tj0 shown in FIG. 9 that the currentincrease from I₁ to I₂ causes the collector-emitter voltage V_(CE) toincrease to V₂. Therefore, a loss of I₂×V₂ is to occur. A junctiontemperature Tj1 is calculated by calculating a temperature increase bymultiplying the power loss I₂×V₂ by the module thermal resistanceRth(j−c) and adding the calculated temperature increase to Tj0.

It is seen from the I_(C)−V_(CE) curve corresponding to the junctiontemperature value Tj1 that the collector-emitter voltage V_(CE) becomesV₃ at I₂. Therefore, the loss is I₂×V₃. A junction temperature Tj2 iscalculated by calculating a temperature increase by multiplying thepower loss I₂×V₃ by the module thermal resistance Rth(j−c) and addingthe calculated temperature increase to Tj0.

It is seen from the I_(C)−V_(CE) curve corresponding to the junctiontemperature value Tj2 that the collector-emitter voltage V_(CE) becomesV₄ at I₂. Therefore, the loss is I₂×V₄. A junction temperature Tj3 iscalculated by calculating a temperature increase by multiplying thepower loss I₂×V₄ by the module thermal resistance Rth(j−c) and addingthe calculated temperature increase to Tj0.

As the above calculation is repeated, the calculated junctiontemperature increases to approach an actual value to occur. As shown inFIG. 10, the junction temperature increase of each calculation decreasesgradually. And a convergence value is considered the actual junctiontemperature Tj of the power semiconductor device 21.

A junction temperature to occur as a result of changing of the currentcan be calculated by repeating the above calculation. Whether theelectric current can be increased may be determined by comparing thecalculated junction temperature Tj and the rated value of the device.

Whereas increase of the junction temperature of the power semiconductordevice 21 takes several seconds, calculations of the first step and thesecond step can be performed by a microcontroller or the like in a veryshort time that is in the order of 0.01 to 0.1 second. Therefore, thereis no problem if these calculations are performed before increasing thetemperature in accordance with an output change instruction. Even if theoutput is changed in accordance with the output change instruction,there is no problem because the calculations are performed in the orderof 0.1 second or shorter.

A junction temperature is not calculated with additional considerationof the contact thermal resistance Rth(c−f) or the heat sink thermalresistance Rth(f−w). This enables more accurate calculations.

The switching loss of the power semiconductor device can be calculatedin accordance with the following equations with an assumption that thevoltage and current change linearly:

on-loss P=⅙ ×V×I×Ton/T

off-loss P=⅙ ××I×Toff/T

where Ton is the switch-on time and Toff is the switch-off time and T isthe cycle.

Therefore, a total loss P may be calculated as the sum of a switchingloss and a steady loss.

According to the embodiment of the invention, damaging of the powersemiconductor device can be prevented by determining an increase ofcollector-emitter voltage V_(CE) due to aging of the power semiconductordevice based on the quantity of heat that is obtained from a temperatureincrease of the cooling water. In addition, deterioration of the coolingperformance of the heat sink due to its aging can also be taken intoconsideration.

In the power conversion apparatus according to the embodiment of theinvention, since a loss of the power semiconductor device 21 is obtainedbased on the temperature increase of the cooling water, the accuracy ofthe calculated loss depends on the difference between temperaturesdetected by the temperature sensors 26 b, 26 c and a flow rate of thecooling water. Thus, the present embodiment is applied to powerconversion apparatus having an output power of about 100 kW or more.

This application is based on Japanese Patent Application Nos.2013-263116 and 2013-263117 both filed on Dec. 19, 2013, the entirecontents of which are incorporated herein by reference.

1. A power conversion apparatus comprising: a power conversion unit; aplurality of sensors attached to the power conversion unit; and acontrol unit configured to control the power conversion unit, whereinthe power conversion unit includes a module having a power semiconductordevice to perform a power conversion and a metal base on which the powersemiconductor device is mounted, and a heat sink arranged to contact themetal base to cool the power semiconductor device, wherein the sensorsare arranged to measure a temperature of the metal base and atemperature and a flow rate of cooling water flowing into and out of theheat sink, wherein the control unit is configured to estimate an initialjunction temperature of the power semiconductor device by obtaining,based on values measured by the sensors, a quantity of heat that flowsfrom the power semiconductor device to the heat sink, wherein thecontrol unit is configured to further obtain, upon receipt of an outputchange instruction to increase power to be output from the powerconversion unit, an updated junction temperature of the powersemiconductor device corresponding to increased power in accordance withthe output change instruction, and wherein the control unit isconfigured to become non-responsive to the output change instructionwhen the control unit determines that the updated junction temperaturereaches a given temperature.
 2. The power conversion apparatus accordingto claim 1, wherein the control unit has device data indicating arelationship between applied electric current and voltage of the powersemiconductor device for each junction temperature, and wherein whenobtaining the updated junction temperature of the power semiconductordevice, the control unit obtains voltage corresponding to an increasedapplied electric current from the device data for the initial junctiontemperature to obtain, based on the increased applied electric currentand the corresponding voltage, a power loss and the updated junctiontemperature corresponding to the power loss, the control unit repeatsthe steps of obtaining updated voltage corresponding to the increasedapplied electric current from the device data for the latest updatedjunction temperature and obtaining, based on the increased appliedelectric current and the updated voltage, updated power loss and anotherupdated junction temperature corresponding to the updated power loss. 3.The power conversion apparatus according to claim 1, wherein theplurality of sensors includes a temperature sensor configured to measurethe temperature of the metal base, wherein the temperature sensor isprovided to contact the metal base or inserted in the metal base, andwherein the control unit estimates the initial junction temperature ofthe power semiconductor device based on a thermal resistance circuitbetween a junction of the power semiconductor device and the metal base.4. A power conversion method for power conversion, the power conversionmethod using a module having a power semiconductor device and a metalbase on which the power semiconductor device is mounted, and a heat sinkarranged to contact the metal base to cool the power semiconductordevice, so as to perform the power conversion by an operation of thepower semiconductor device, the power conversion method comprising:estimating an initial junction temperature of the power semiconductordevice by measuring a quantity of heat that flows from the powersemiconductor device to the heat sink when electric current is appliedto the power semiconductor device; obtaining, when the electric currentapplied to the power semiconductor device is to be increased, an updatedjunction temperature of the power semiconductor device based on anincreased applied electric current; and withholding an increase of theelectric current applied to the power semiconductor device when it isdetermined that the updated junction temperature reaches a giventemperature.
 5. The power conversion method for power conversionaccording to claim 4, wherein the obtaining the updated junctiontemperature of the power semiconductor device includes obtaining voltagecorresponding to the increased applied electric current from devicecharacteristics of the power semiconductor device based on the initialjunction temperature to obtain, based on the increased applied electriccurrent and the corresponding voltage, a power loss and the updatedjunction temperature corresponding to the power loss, and then repeatinga series of steps of obtaining an updated voltage corresponding to theincreased applied electric current from the device characteristics ofthe power semiconductor device based on the latest updated junctiontemperature, and obtaining, based on the increased applied electriccurrent and the updated voltage, an updated power loss and anotherupdated junction temperature corresponding to the updated power loss. 6.The power conversion method for power conversion according to claim 5,wherein the series of steps are repeated until the updated junctiontemperature converges.